1. Field of the Invention
The present invention relates to a sliding bearing and a manufacturing method thereof, and more particularly, to a sliding bearing having a sintered alloy layer formed on a steel-backed metal layer by sintering a compacted body made of iron-based metal mixture powder and concurrently bonding the same to the steel-backed metal layer so that load-carrying capacity (permissible load), fatigue strength, and fitting characteristic in the housing are improved, and also having lubricating oil meeting the optimal lubrication conditions impregnated therein so that a grease refilling period is considerably extended, facilitating maintenance of the sliding bearing, and a manufacturing method of the same.
2. Description of the Related Art
In general, a sliding bearing is a machine part for supporting a shaft, a journal, or a counter plate used for a driving or sliding portion of industrial machines such as construction equipments of excavator, forklift, crane, machine tools, presses, injection machines, or vehicles. The sliding bearing can be classified into various types such as a bush type, a half-pipe type, and a plate type, and usually formed by bonding copper-based alloy, aluminum-based alloy, copper-lead-based alloy, or copper-based and synthetic resin-based complex material to a steel-backed metal layer.
The structure of each conventional sliding bearing will be described as follows.
First, a sliding bearing formed by bonding an aluminum-based alloy layer to a steel-backed metal layer includes a steel-backed metal layer, an intermediate layer attached to the steel-backed metal layer, and an aluminum bearing alloy layer attached to the intermediate layer. The aluminum alloy layer includes a small amount of tin (Sn) and silicon (Si), and the intermediate layer is formed of an aluminum alloy layer including a small amount of one or more elements selected from the group consisting of manganese (Mn), copper (Cu), and magnesium (Mg).
Second, a sliding bearing having a copper-lead-based alloy layer has a multi-layered structure composed of a steel-backed metal layer, a copper-lead-based bearing alloy layer combined to the steel-backed metal layer, and a tin-containing lead alloy combined to the bearing alloy layer.
Third, there is a sliding bearing having a bearing layer formed of synthetic resin as a main ingredient which is formed on the surface thereof. In this sliding bearing, a porous metal layer is formed on an inner metal layer such as a steel plate and a bearing layer having lubricating synthetic resin as a main ingredient which covers a surface of the porous metal layer and part of which is impregnated in the pores of the porous metal layer. The bearing layer is formed by solidifying 4-ethylene resin fluoride and solid lubricant so that fine powder particles of the PTFE resin and the solid lubricant cohere each other.
However, the conventional sliding bearings having the above structures have demerits. That is, since the alloy layers contacting the counterpart of the sliding bearing are formed of copper-, copper-tin-, copper-lead-, aluminum-, or synthetic resin-based material exhibiting a relatively low hardness, it is not appropriate to use these sliding bearings for the permissible load over 300-500 kgf/cm.sup.2. Also, since it is not possible to increase strength and hardness through heat treatment, wear resistance of the sliding bearing is improved.
To overcome the above problems, as a sliding bearing, a sintered alloy bearing has been used having only an iron-based sintered alloy layer without a steel-backed metal layer. However, the sintered alloy bearing is only used in a small size, i. e., the diameter and length of the bearing are within a range of 5-50 mm, since it is considerably difficult to make the large size sintered bearing have the high density. Furthermore, when solid lubricant is included in the sintered alloy bearing and the high pressure for molding (compacting) is applied to increase the density of the bearing, the solid lubricant acts as an impediment to the sintering so that the strength and load-carrying capacity of the bearing is further reduced. Thus, with the iron-based sintered alloy layer only, low friction and high load-carrying capacity (or high permissible load) cannot be simultaneously achieved.
To solve the above problems, a sliding bearing in which a steel-backed metal layer is bonded to an iron-based sintered alloy layer has been proposed. Here, the sintered alloy layer is required to include at least an iron component of 70 wt % or more and porosity of 10-30%, to satisfy conditions of a predetermined load-carrying capacity, high resistance to fatigue strength and fitting characteristic in the housing, prevention of detachment from the housing, and prevention of oil loss through a rear side. However, a sintered alloy layer satisfying such conditions is not easily overlaid onto the steel-backed metal layer since, when an iron-based mixture powder body of such composition is sintered, the mixture powder body severely shrinks due to densification phenomenon. Also, since the steel-backed metal layer expands at the corresponding sintering temperature, the iron-based sintered body and the steel-backed metal layer are not bonded to each other and thus the bearing is manufactured in a state the iron-based sintered body and the steel-backed metal layer being separated.
Due to the difficulties in bonding the steel-backed metal layer and the sintered alloy layer, in order to manufacture a sliding bearing of which an iron-based sintered alloy layer is bonded to a steel-backed metal layer, a sintered alloy layer is first formed by sintering iron-based metal alloy powder and then the sintered alloy layer is bonded to a steel-backed metal layer by a method such as welding or brazing. In this case, however, a process for manufacturing the sliding bearing is divided into two steps and further pores can be clogged by a filler metal fused during brazing which is easily absorbed in the pores of the sintered alloy layer due to capillary action. As a result, it is difficult to obtain porosity of 10% or more in volume in the sintered alloy layer for impregnation of the lubricant, and brittle structures are formed due to a reaction between the fused filler metal and the sintered alloy so that the sintered alloy layer is brittle against impacts. Also, flux used for brazing remains in the pores of the sintered alloy layer so that the sintered alloy layer can be easily and rapidly corroded, causing a bad effect of the lubricating property of the sliding bearing.
Meanwhile, for the best performance of the lubricant of the sliding bearing, the sintered alloy layer must be impregnated with lubricating oil. However, since the best conditions of impregnation of the lubricating oil for the sliding bearing having an iron-based sintered alloy layer sinter-bonded to a steel-backed metal layer has not been known so far, the performance of the lubricant can not be maximized.
Further, since grease must be frequently refilled as necessary after the conventional sliding bearing is initially installed, the maintenance and management of the sliding bearing is difficult and inconvenient.